The remnants of an enormous fish kill that left millions of Atlantic menhaden strewn near Masonboro Island are now limited to a few thousand fish carcasses, piled and draped among marsh grasses on the island’s north side. The pungent stench of dead fish still pierced the air along the beach recently, but pelicans and seagulls, clustered by the hundreds nearby, had disposed of the majority of the remains.

“After it happened, the fish were piled thick enough to be visible from the air,” said Byron Toothman, a research biologist with the N.C. Coastal Reserve and National Estuarine Research Reserve. “This may look bad, but it’s nothing like it was.”

The event, which left meter-deep piles of the small, silver fish along estuarine shorelines throughout New Hanover County, occurred in the early morning hours of Jan. 8 and was first discovered by beachwalkers. Data from a nearby water-quality monitoring station showed that levels of dissolved oxygen in the water decreased by roughly 90 percent in just two hours, an enormous drop, experts said.

“Before 1:15 a.m., the dissolved oxygen concentration was between 96 and 97 percent. By 3:15 a.m., it was less than 10 percent,” said Heather Wells, a research biologist with the program. “It’s an extremely significant decrease.”

Though the monitoring station, one of four in the Cape Fear region, has been in place since 1995, this month’s fish kill was the first to occur near enough to collect pertinent data in a timely fashion. Biologists believe, based on the location of the menhaden and the quick drop in oxygen levels, that the fish swam into the inlet by mistake, became confused and couldn’t swim out before the tide turned and the oxygen ran out.

“Menhaden school very densely,” Toothman said. “It’s likely that they got tied up and couldn’t figure out a way out.”

Of course, researchers don’t know for certain what caused the fish kill, but having a complete data set from the moment the menhaden began to die helps rule out some possibilities.

“Initially, we were examining the fish to see if they had sores or some identifiable sickness,” Wells said. “We’re continuing to piece it together. The data gives a more definite answer.”

Each monitoring station measures a handful of water-quality markers — including temperature, salinity, pH and turbidity — in 15-minute intervals. The monitors — heavy cylinders with a handful of sensors at one end, encased in a copper cap — hang from ropes enclosed in large PVC pipes that are attached to pilings throughout the estuaries. Two of the stations provide data in real time via satellite, but researchers can only access information collected by the other two by boating to the various station locations and swapping the cylinders out, then uploading the information to computers at a lab on the University of North Carolina Wilmington campus.

Page 2 of 2 - Collected over a long period, the data allows researchers to examine significant events on multiple time scales, which can help establish long-term water-quality trends against the backdrop of historical information.

“Having that long-term monitoring is what helped us narrow it down,” he said. “Sometimes we can’t explain why these things happen. It’s because of the data collection that we’re able to more precisely say what caused the fish kill.”